This invention relates generally to arc discharge flash lamp systems and, more particularly, to a more efficient multiflash system.
Multiflash systems are employed in a variety of applications; for example, reprographic machines; laser excitation; and warning flashers on airplanes, towers, road barriers, marine equipment and tower monitored lighting systems for airport runways.
Flash tubes generally comprise two spaced apart electrodes within an hermetically sealed glass envelope having a rare gas fill, typically xenon, at a subatmospheric pressure. Such lamps are connected across one or more storage capacitors charged to a substantial potential, which is, however, insufficient to ionize the xenon gas fill. Upon application of an additional pulse of sufficient voltage, the xenon is ionized and an electric arc is formed between the two electrodes, discharging the capacitor through the flash tube, which emits a burst of intense light. In many cases the pulse voltage is applied between an external trigger wire wrapped around the envelope and the electrodes; this is referred to as shunt triggering. However, in other cases an external wire is not feasible since it may result in an undesirable arcing between the trigger wire and a proximate lamp reflector or else the high potential applied to the external trigger wire might be hazardous to operating personnel. In those cases, the lamp may be internally triggered by applying the pulse voltage directly across the lamp electrodes, a technique referred to as injection triggering. Usually the voltage required is about 30 to 50 percent higher than that required to trigger the same lamp with external trigger wire, and the trigger transformer secondary must carry the full lamp current.
In applications requiring two (or more) flash lamps, the lamps have been series-connected across the storage capacitor means, with a single injection trigger circuit being used for the series lamp combination. Whether using one lamp or a plurality of lamps, the general operation of the prior flash circuits comprised charging the storage capacitor means; typically through a resistor, to a predetermined level of voltage, then, on command, triggering the lamp (or lamps) into ionization and thereby discharging the capacitor means through the ionized lamp (or lamps). The energy thus developed in the lamp (or set of lamps) is equal to one-half of the capacitance of the storage means multiplied by the square of the charged voltage. Accordingly, this conventional method of operation results in the waste of a considerable amount of energy in charging the storage capacitor means through a power dissipating resitor. Further, time is wasted in "coming up to charge", or the storage capacitor means must be maintained in a fully charged state until called upon to flash the lamp.
One approach for overcoming the aforementioned shortcomings of conventional flash lamp arrangements is described in a copending application Ser. No. 865,405 filed concurrently herewith and assigned to the present assignee. Briefly, the operating circuit of this copending application uses the charging current of the storage capacitor as well as the discharge current for purposes of lamp energization. More specifically, first and second arc discharge flash lamps are series connected across a supply voltage source comprising a direct current storage bank. The storage capacitor is connected between the junctions of the lamps and one terminal of the source. Respective injection or shunt means are provided for coupling trigger pulses to each lamp, and a succession of high-voltage trigger pulses are alternately applied through the respective coupling means to the lamps. Each trigger pulse applied to the first lamp effects an arc path therethrough for charging the capacitor, and each trigger pulse applied to the second lamp effects an arc path therethrough for discharging the capacitor. Hence, the storage capacitor is charged through one lamp and discharged through the other in response to trigger pulses, which are applied in alternate sequence to the lamps. In essence, the lamps function as alternately actuated switches for charging and discharging the capacitor.
The flashes can be synchronized so that the human eye cannot perceive any variation in time between the flashes, e.g., four milliseconds between flashes. Such multiflash capability for predetermined durations is particularly useful for reprographic applications.
Efficiency is significantly increased by the elimination of power dissipating and time consuming charging resistors. The capacitor means delivers approximately twice the normal power to the lamp by virtue of its charging current as well as its discharge current. Accordingly, the capacitance for a given multiflash system in which the charge cycle is used for lamp energization, as well as the discharge cycle, may be approximately one half that required for the storage capacitor of a comparable system (i.e., same voltage and joule rating) employing a conventional resistor charge circuit. As a result, the circuit permits the use of a smaller capacitor with attendant reductions in cost and package size.
Further, the tendency of the arc discharge to hang on is reduced as each lamp functions as a switch, and the buildup of the voltage on the storage capacitor with respect to the source causes the first lamp (during the charge cycle) to extinguish at the proper time. During the discharge cycle, the second lamp extinguishes due to the limited energy capacity of the storage capacitor with respect to the source.
The present invention provides a multiflash system concept which incorporates principles of the above-mentioned copending application but provides significant advantages in efficiency, cost and performance.